I'm not sure which plugin version are you using, but the current one (4.5.1) has a new feature called “Index table engine”. It was designed mainly for databases with high record counts, to minimalize the SQL query CPU needs.
There are 4 new chapters in the documentation about the index table usage, you can check them out.
Also, there are 3 performance tuning chapters in the documentation, that will definitely help you optimizing the plugin performance.
To safely update the plugin, please read the important update notes.Best,
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